IMPRESSION TECHNIQUES IN IMPLANTOLOGY

Introduction :

The implant impression is the first link in the chain of production of the supra-implant prosthesis. It is the most decisive step in the success of the manufacture of fixed supra-implant structures and therefore their durability.

DEFINITION: according to:

Goudot 2013 “ The implant impression is an impression of situation or positioning (dimensional precision is sought). It is opposed to impressions in fixed dental prosthesis which is an impression of morphological reproduction where precision of detail is sought.”

So: The implant impression is a technique which allows the POSITIONING of the implant to be TRANSFERRED PRECISELY , with the aim of creating a working model for the production of the prosthesis.

Principle and objectives of the implant impression:

In implantology, dimensional accuracy is more important than that of recorded surface conditions, which differentiates the objectives of the impression from that intended for the prosthesis fixed on natural teeth.

The impression in implantology first allows the precise position of the implant to be recorded in the three planes of space thanks to a transfer system, but also the shape of the surrounding soft tissues and then this information to be transferred onto the working model using the transfer, the replica and a silicone representing the false gum.

Indeed, when using impression transfers and implant replicas, the accuracy of adaptation of the implant with the prosthetic connection is determined by the quality of the machining and does not depend on that of the impression. The impression is therefore a positional impression.

Comparative table of specifications for impressions in implant prostheses and conventional prostheses

Specifications for impressions in tooth-supported prostheses	Specifications for impressions in implant-supported prostheses
Recording the morphology of the abutment tooth which is unknown. Recording the axial and occlusal surfaces of the abutment tooth. Recording the cervical limits of the abutment tooth. Recording the emergence profile. Recording the proximal surfaces of the teeth adjacent to the abutment tooth. Recording the anatomy of the remaining teeth. Recording the tissue environment.	The morphology of the implant or abutment is known, less precision in recording details. Recording the position of the implant / abutment. Recording the tissue environment. Recording the proximal surfaces of the teeth adjacent to the implant. Recording the anatomy of the remaining teeth.

Specificity of the implant impression : The implant impression owes its specificity in principle to the difference that exists between a tooth and an implant.
- It is a positioning impression, the machined prosthetic surfaces are known and determined in advance, they only require to be repositioned on a working model.
- The implant impression must take into account the particularities of the implant prosthesis, in particular its machined and ankylosed support.
- The mobility of the implant support is less than the mobility of the natural tooth, it is only around 10µm : this is why this type of impression requires great dimensional precision but less precision in the recording of details compared to an impression of natural teeth.
- In supra-implant prosthetics, the main difficulty is to obtain a passive adaptation of the prosthetic elements on the implants. Therefore, the precision of the recording of the spatial positions of the implants is crucial.

A defective impression leads to a lack of passive adaptation of the over-implant prosthesis and generates constraints that can cause fractures of the ceramic, screws or the implant itself. They can even induce a loss of osseointegration.

Prerequisites for the implant impression:
Confirmation of osseointegration:

For the implant to be successful, the replacement tooth and bone must fuse together, through a process known as osseointegration.

It is essential to check the proper bone integration of the implants before starting the prosthetic construction. This validation is generally done according to the success criteria established by Albrektsson in 1986, clinically and radiologically using several tests.

Absence of painful infectious syndrome.
Absence of peri-implant radiolucency, a sign of fibro-integration.
Percussion test on the healing pillar which gives a clear sound.
Total immobility of the implant: tested either by:
- A counter torque of 35 Ncm with a torque wrench on the implant.
- Devices such as the Osstell, the Periotest.

Impression materials and equipment:

Impression Tray : A good impression starts with a good impression tray. The choice of impression tray depends mainly on the type of impression you want to make.
- Perforated impression tray opposite the implant site in the case of an open impression.
- Non-perforated impression tray in the case of closed impression.

Whatever impression technique is chosen, it must allow good adaptation to the patient’s oral situation and a rigidity allowing increased precision and better homogeneity of the thickness of the material.

It could be a:

Individual impression tray, perforated or not, made of resin.
Commercially available impression tray, made of polycarbonate, plastic, or metal. It can be perforated or not.

Figure: Impression trays. From left to right: individual resin impression tray, commercial impression tray in perforated metal, commercial impression tray in perforable plastic, commercial impression tray perforated.

The Transfer (imprint pillar):

Is a part machined with great precision to fit unambiguously on the head of the implant and its analogue (or replica), it serves to transfer the position of the implant in the impression. It can be screwed or clipped onto the part to be recorded (implant or machined prosthetic abutment).

The impression transfer is thus chosen according to the diameter of the implant and its connection and the emergence profile of the healing screw. It is also chosen according to the impression technique chosen.

Implant analogues (implant replicas):

This is an element that screws onto the implant transfer and allows the implant positioning to be reproduced on the plaster model. This implant analogue must also be planned before the impression and corresponds to the replica of the implant placed.

Impression materials

In implant-supported prosthetics, the ideal impression material must have sufficient fluidity to print peri-implant structures, while having excellent dimensional stability and sufficient rigidity.

The materials must therefore meet delicate specifications:

Elasticity to allow recording of undercut areas.
Dimensional accuracy with good elastic recovery capacity after deformation.
Dimensional stability and mechanical strength after shrinkage.
Viscosity compatible with recording of peri-implant soft tissues.
High final hardness to ensure unambiguous removal or replacement of the transfer.
Easy to handle, sufficient preparation time and short setting time. Three main families of materials coexist:

Elastomers (Polyethers, Silicones A and C, Polysulfides)
The casts
Hydrocolloids

Polyethers and polyvinylsiloxanes (Silicones A) are the reference materials for impressions in implantology (viscosity, dimensional stability, hardness)
Plaster is the material of choice for the impression of the completely edentulous person
Reversible hydrocolloids are rarely used in implantology due to their fragility. They are used for preliminary impressions or for impressions of the opposing arch.

Impression techniques

Impression techniques are classified as direct and indirect. Impressions can be taken using either an open or closed tray; the choice of impression transfer depends on the method chosen.

Indirect technique (repositioned, closed-air, Reynolds, pop-in, pop-up or twist-lock technique)

The indirect technique is simpler and more similar to traditional impressions. Impression transfers are generally conical-shaped reliefs screwed onto the implant collar or intermediate abutment and are not carried along in the impression. They remain in place, screwed to the implants or abutments.

In a second step, these transfers are screwed onto an implant or abutment analogue and then reinserted into the impression before processing.

Surface sculptures, in the form of grooves and lunules in the impression, allow for the most precise repositioning possible.

Protocol:

The healing screw or temporary crown is removed from the implant head.
The transfer is screwed onto the implant head using a screwdriver or manually.
A control X-ray is performed to check the correct adaptation of the transfer base and implant neck.
Checking the impression tray in the mouth (commercial or individual).
Filling the impression tray with the high viscosity material, and injecting the low viscosity material around the impression transfer.
Inserting the impression tray.
Removal, verification and decontamination of the impression (the transfer remains screwed to the implant).
Unscrewing the transfer and immediately screwing the healing screw back onto the implant so as to prevent soft tissue from covering the implant if it is deeply buried.
careful joining by the practitioner, outside the mouth, of the impression transfer and the laboratory analog
very meticulous repositioning of the transfer assembly and analogue in the silicone. This step is often the source of errors.
Impression of the opposing arch.

The different stages of making a closed-air impression

healing pillar in place.
Implant summit after deposition of the healing abutment
Twist-lock transfer in place
X-ray control.
Footprint after removal.
Sectional impression, without transfer
Twist-lock transfer and its transfixation screw, and implant analogue
Analog screwed onto the twist-lock transfer repositioned in the impression

Indications	Benefits	Disadvantages
Mixed impressions involving both dental preparations and implants. Nauseous patient who can thus avoid the delicate maneuver of unscrewing the transfers. Small mouth opening. Slightly divergent implants. Fragile gums.	Simplicity and speed of execution. Low mouth opening height required. Allows the transfers and healing screws to be placed and removed one by one.	According to some authors, the precision would be less good, risk of imprecision during repositioning in the impression.

Direct technique (carried away, open or Pick-Up technique) : Consists of using screw-retained transfers designed to be retentive. They are undercut and must not be able to be removed from the impression; sharp edges prevent any rotation in the impression when screwing the analog.

The impression tray used (commercial impression tray or individual impression tray) is perforated opposite the location of the impression transfer.

Protocol:

unscrewing the healing screws with a manual screwdriver adapted to the implant system used
The transfer is screwed into the implant using a screwdriver or manually. A wooden cone seals the screw head to prevent the impression material from leaking inside.
A control X-ray is performed to check that the transfer is correctly adapted to the implant.
preparation of an openwork impression tray at the level of the transfer screw passage.
If the number of implants is large, they can be secured together in the mouth using self-adhesive resin (Duralay®) or photo-polymerizable resin alone, or after placing dental floss or a metal rod (modified direct technique).
check in the mouth, before placing the silicone, that the walls of the impression tray do not interfere with the impression transfers
Filling the impression tray with the high viscosity material, and injecting the low viscosity material around the impression transfer .
Inserting the impression tray. It is important to push the impression tray in far enough so that the transfer screw emerges from the material and can be unscrewed.
After the material has set, the impression is removed after unscrewing the transfers.
re-tightening the healing screw or abutment cover cap
bonding of the laboratory analogue to the impression transfer, through silicone
imprint of the antagonistic arch.

The different stages of making an open-air print (108)

healing pillar in place.
Implant summit after deposition of the healing abutment
The transfer in place
X-ray control.
Transfer cover to prevent light from penetrating the screw access holes
Trying on the openwork impression tray
Footprint after removal.
Setting up the analogue
Analog screwed onto the transfer

Indications	Benefits	Disadvantages
Numerous abutments. Divergent implant axes. Deeply buried implants.	Reduced stress when removing the impression. More precise.	The height of the transfers makes the technique unusable if the mouth opening is insufficient. It is necessary to window the impression tray. Inserting the impression tray is delicate (in relation to the window). Risk of inaccuracy when screwing it back onto the analog. Requires simultaneous placement and removal of all transfers and all healing screws… a delicate and sometimes painful step if the embedding is deep.

Variants:

the technique with clipped transfers (snap-on or coping impression): It uses plastic transfers clipped onto titanium pillars (snap)

Implementation steps:

Removal of the healing abutment (a,b) .
Screwing the titanium snappy abutment (c).
Place the plastic clip on the pillar (fit well) (d).
Filling the pillar with a light silicone (e).
The plastic clip comes with the print when it is removed (f).
Placement of the abutment analog in the clip taken in the impression (g,h).

This technique is very similar to the direct technique with an open impression tray because the clip-on transfers are taken into the impression and do not need to be repositioned. However, the abutment analog must be clipped onto this plastic part contained in the impression. This step is delicate because the force required for the fitting is likely to deform the impression material.

Different stages of making a snap-on print

Customized impression for aesthetic purposes (emergence profile transfer technique) In aesthetic situations, the use of conventional impression transfer cannot give satisfactory results without prior modification. The gingival profile is not in line with the trans-gingival part of the impression transfer. The temporary crown models the gum and determines a certain emergence profile. When the latter is satisfactory from a cosmetic point of view,

aesthetic, it may be desirable to reproduce it identically for the crown of use. For this we can use two simple techniques:

The first technique: consists of transforming the impression transfer by adding photopolymerizable fluid composite.

Implementation steps: after validation of the emergence profile of the temporary crown:

an implant analogue is screwed onto the temporary crown
The analogous temporary crown assembly is inserted into a silicone block with a very high shore hardness.
Once the silicone has set, we unscrew the temporary crown to screw an impression transfer onto the implant replica.
fill the space left by the absence of the temporary crown with light-cured flowable composite.
recovery of the transfer and the composite marking the emergence profile.
placement of the modified transfer in the mouth on the implant.
The resin replaces the soft tissues that have collapsed after removal of the temporary crown.
radiographic control
the double-mix impression is taken
impression removed with the resin which marks the emergence profile
The double-mix impression is taken and an implant replica is screwed onto the impression transfer.
The prosthetics laboratory can then cast the impression with plaster and a false gum to obtain a working model that perfectly reproduces clinical reality.

The impression transfer is screwed onto the head of the implants. It can be noted

the space between the transfer and the gum

A replica implant is screwed onto the temporary crown and then inserted into silicone

The print transfer is

screwed onto the implant replica then fluid composite is injected and photopolymerized.

The modified impression transfer is screwed onto the implant head and the double-mix impression is taken

emergence profile transfer technique By modification of the impression transfer

The second technique is different in that the print transfer is not modified.

Stages of implementation:

The transfer is screwed onto the head of the implant;
Radiographic control of correct positioning;
A fluid silicone is injected around the transfer to fill the available space
We let the silicone finish setting before making the classic double-mix impression.
The pre-injected silicone prevents the gum from closing under the effect of the compression of the silicone during the double-mix impression. This technique has the advantage of being very fast, very reliable and does not require transforming the impression transfer.

The impression transfer is screwed in and fluid silicone is injected to fill the available space and hold the gum in place while the full arch impression is taken.

The double-mix impression is taken and an implant replica is screwed onto the impression transfer.

The criteria for choosing the different techniques:

Mouth opening : Pickup impressions, requiring the use of a screwdriver for insertion and removal of the impression, will be contraindicated in cases of limited mouth opening.
The gag reflex: The impression in a nauseous patient must be able to be disinserted quickly, which contraindicates techniques where the impression is screwed in (Pickup).
The implant axes:
- If the implants are parallel, the removal of Pickup or twist lock transfers directly onto implants or abutments is easy.
- If the implants have a divergence of more than 8°, the forces required to disengage the impression will cause a constraint that will generate inaccuracy in the impression. An indirect impression will therefore be impossible if the implant axes are too divergent.
The number of implants:
- from 1 to 3 implants: the impression techniques are equivalent in terms of precision and the practitioner will rely on other criteria for his choice,
- 3 implants: the need for an impression validation key directs the choice towards a Pickup technique.
Burial depth : Footprint accuracy decreases with burial;
- In the case of deeply buried implants (≥ 2 mm), the practitioner will therefore opt for a Pickup technique where the sensation of progressive screwing will counter the resistance of the gingival tissue, while the radio-opacity of the metal transfer will allow radiographic control.
- Beyond 4 mm, apical extension of the transfers is necessary.
Loading: In the case of direct loading and at the time of the impression the patient is tired, the mouth opening becomes increasingly weak. It is therefore interesting to use Pop in impression techniques which offer speed of execution and satisfactory precision.

e – Validation and passivity key: Sheffield key

One of the criteria for the success of implant prostheses is the passivity of the framework. To achieve the passivity of this framework, it is necessary to be sure that the model transmitted to the laboratory technician is entirely faithful.

This particular assembly is called an implant impression validation key .

This key is sent to the practitioner who made the working impression. He will try the key by screwing each transfer, preferably with the opposite screwing method as for validating a framework.
If the plaster resists, it means that the three-dimensional reproduction of the implants in the impression is good and the impression is validated.
If the plaster cracks, the impression will have to be redone because it is false.

Validation key and passivity

Optical or digital fingerprints:

The optical impression uses a process that exploits light to record the dental arch in three dimensions: The optical impression on implant aims at digitization, that is to say the three-dimensional recording of the position of the implant in the mouth. Indeed, after modeling (three-dimensional computer graphics step which consists of modeling a 3D object in software), we obtain the sub-gingival situation of the implant within the dental arch.

Protocol

The healing abutment or temporary prosthesis is unscrewed and the practitioner, using his camera, attempts to record the gingival cradle and the mesial and distal surfaces bordering the edentulous area.
In a second step, after selecting the position of the implants, the clinical procedure being today similar to all cameras, the scanning body is screwed or clipped the intra-oral optical impression is carried out.
In a third step, the antagonistic arch is recorded.
fourthly, take the intraoral optical impression in the maximum intercuspation position (MIP), allowing a correlation of the maxillary arch with the mandibular arch

Optical fingerprint

The advantages of optical impression

For the patient: absence of unpleasant sensations (gag reflex, bad taste, etc.), possibility of communication making the patient “an actor” in their therapy.

For the practitioner: accuracy of measurements for small to medium-sized teeth, simple learning, less time-consuming technique and lower overall cost due to shorter working time

Imprint processing:
1. Disinfection of the impression:
  1. in the office, rinse thoroughly with tap water
  2. spraying of a preparation based on glutaraldehyde at 2

% for polyethers or 2.5% sodium hypochlorite disinfectant solution For vinylpolysiloxanes

Making a false silicone gum

Before pouring the impression, the prosthetist creates a removable silicone false gum to reproduce the peri-implant soft tissues . Silicone, which is less hard than plaster, allows:

To understand the height of the peri-implant gum.
To prepare a gingival emergence profile adapted to the tooth to be restored.
A framework design in accordance with periodontal health.
To easily access implant replicas.

In fact, this silicone is poured directly into the impression, around and up to the neck of the pillar, and must have a thickness of at least 2mm.

Casting the impression

After casting the silicone false gum, the prosthetist casts the extra-hard type IV plaster model.

Conclusion

Analogues in place; silicone false gum is injected Working model completed

Taking an impression in implantology is a fundamental step that determines the entire rest of the treatment. Indeed, if there is an error at this stage, all subsequent steps will be incorrect. The various techniques mentioned offer the practitioner the opportunity to choose one technique or another depending on the clinical situation presented to them.

Bibliography

Davarpanah M. / Clinical Implantology Manual 3rd edition. CdP, 2012.
M.BERT, P.MISSIKA ./ The keys to success in implantology. Edition CdP, 2009
T.DEGORCE /The impression in fixed implant prosthesis Prosthetic strategy June 2002
T.DEGORCE / Implant impression and fixed prosthesis impression, what are the differences?

Prosthetic Strategy February 2005

N.Attard, I.Barzilay /Modified impression technique for accurate recording of peri-implant soft tissues. Journal of the Canadian Dental Association 2003
E. CLAVEL, J. PENAUD, J. SCHOUVER, JF. CLAVEL ; From the impression to the working model in supra-implant prosthesis. DENTAL INFORMATION 2012
Y. BENHAMOU, Y. ALLARD, Y. CHARBIT, P. MAHLER Impression taking in prosthetics on implants: problems and errors. Prosthetic strategy September-October 2010
Arnaud Soenen . Intraoral optical impression in implant prosthesis DENTAL INFORMATION January 2018